Motor having single cone air dynamic bearing balanced with shaft end magnetic attraction

ABSTRACT

A single cone air dynamic bearing motor, including a shaft having a diminishing conical taper surface, a sleeve having a concavity opposite the shaft, and magnetic members to generate magnetic attraction between the shaft end and the sleeve. Grooves are formed on the conical taper surface of the shaft or the sleeve so as to create load capacity when the motor rotates, whereby rotating parts of the motor are supported by the axial components of the load capacity balanced with the magnetic attraction. The motor thereby achieves reduction in thickness, current, and cost, and inhibits non-repeatable runout.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to an air dynamic bearing motor, and moreparticularly to an air dynamic bearing having a conical shape to enablethe motor to be smaller in thickness and lower in cost.

2. Description of the Related Art

There has been a trend toward the fluid dynamic bearing motor as thepower source for rotary memory devices, cooling fans, and the like,because of its quietness in operation and the necessity to reducenonrepeatable runout (NRRO) of rotating parts. Portable applications ofsuch electronic devices have been widespread, increasing the demands forfurther reduction in their thickness and required current. However,there are limitations on further reduction in thickness of the fluiddynamic bearings, because they need to have a certain span between thebearings for supporting the shaft in order to inhibit NRRO. Also, inorder to maintain a constant clearance between the bearings, they mustbe machined with extreme precision in the order of submicrons, wherebyit is difficult to produce them at low cost.

In order to make fluid dynamic bearings thinner, a novel structure isnecessary which does not require two bearings for supporting the shaftat axially spaced positions. The bearings should have as little slidingarea as possible so as to achieve a reduction in the required current.Further, cost reduction will be achieved through the development of astructure wherein the bearing clearance is maintained with necessaryaccuracy even with the components machined with a lower degree ofprecision.

Single cone fluid dynamic bearings, which can support loads of bothradial and thrust directions, have attracted attention as havingpotentialities in many respects. However, while some single conestructures that help decrease the thickness of the bearing have beenproposed, for example, in Japanese Laid-open Utility Model PublicationNo. Hei. 06-004731, these are for air dynamic bearings and anyway havenot been very successful. The main reason is that the single conebearing is structurally incapable of sufficiently inhibiting NRRO duringrotation. Japanese Laid-open Patent Publications No. 2000-004557 and No.2000-205248 propose combined use of a conical bearing and a cylindricalbearing to improve the overall performance. However, the cylindricalbearing requires high-degree machining precision for maintaining aconstant bearing clearance, canceling out the advantages of the conicalbearing. U.S. Pat. No. 5,854,524 discloses a single semi-spherical airdynamic bearing having a similar structure as that of the single conebearing, but in this case also, the radius of two spherical surfacesmust be strictly controlled to secure a sufficient radial load capacity,because of which cost reduction is hardly achievable.

Thus the problems yet to be resolved in single cone fluid dynamicbearing motors are how to improve the stability in its rotatingattitude, and how to realize a structure which is easy to assemble.

SUMMARY OF THE INVENTION

An object of the present invention is to resolve these problems and toprovide a single cone air dynamic bearing motor which can be reduced inthickness and required current, and is simple and can be produced atlower cost.

An air dynamic bearing motor according to the invention includes a shafthaving a diminishing conical taper surface, a sleeve having a conicalconcavity opposite the shaft, and means for generating magneticattraction between one end of the shaft and a cone apex of the sleeve.In this construction, a plurality of grooves are formed on a conicaltaper surface of one of the shaft and the sleeve, and the grooves areprovided for creating load capacity when the motor rotates, wherebyrotating parts of the motor are supported by axial components of theload capacity balanced with the magnetic attraction.

The means for generating magnetic attraction includes a permanent magnetand a magnetic material, respectively provided inside the shaft and inthe apex of the sleeve opposite to the shaft, or vice versa. Magneticattraction developed at the end of the shaft acts on the shaft to adjustits position in cooperation with the load capacity created by thegrooves, thereby ensuring the stable attitude of rotating parts.

A ring-shaped member is fixed to one end of the sleeve, and an annularrecess is provided in the fixed member, the free edge of the ring-shapedmember being positioned within the annular recess, so as to restrict anaxial movable distance of the rotating parts. This structure serves as astopper for the rotating parts in the case where the motor is subjectedto a large shock.

Moreover, the following structures for a permanent magnet to protrudefrom one end of the shaft are proposed. The shaft includes the permanentmagnet held inside. The permanent magnet is assembled with the shaftsuch that it is initially held movably but firmly enough to overcome themagnetic attraction as being substantially protruded from one end of theshaft, and is pressed into the shaft by a pressure larger than themagnetic attraction applied from both ends of the shaft and the sleeveto a predetermined position, where the cone apex of the sleeve or aplate spring interposed between the apex of the sleeve and the permanentmagnet is resiliently deformed, whereby when the motor is stationary thepermanent magnet and the apex of the sleeve or the plate spring makecontact with each other, while they are brought out of contact when themotor is rotating, by a distance equal to or shorter than an axialflying height determined on conical surfaces of the shaft and sleeve.Thereby, the start-up failure caused by the conical surface of the shaftbeing fitted in the sleeve when the motor is not in operation can beavoided, improving the reliability of the motor.

Alternatively, the grooves may be formed on both opposite taper surfacesof the shaft and the sleeve at the almost same axial positions. In thisconstitution, the grooves have different angular length from each otherin the circumferential direction. Thereby, each delay, from the timepoint when the bearing clearance becomes small until the time point whenthe pressure in the air in the clearance becomes local maximum by thecorresponding groove, is varied in proportion to the correspondingangular length of each of the grooves. Thereby, an improved constitutionwhich can avoid half whirls and other unstable movements of the motorcan be provided.

According to the air dynamic bearing motor of the present invention, theload capacity created by the rotation of the motor acts vertically withrespect to the conical surfaces, causing the shaft and the sleeve torotate in non-contact relationship at a position where the axialcomponents of the load capacity and the magnetic attraction are inequilibrium. The radial components of the load capacity counterbalanceeach other at respective circumferential points, thereby contributing tothe centering of the rotating parts. The load capacity itself actsvertically on the tapered surface of the cone, and therefore it servesto adjust the attitude of rotating parts when they tilt with respect tothe fulcrum conforming to the cone apex. Magnetic attraction developedat the end of the shaft acts on the shaft to adjust its position incooperation with the load capacity created by the grooves, therebyensuring the stable attitude of rotating parts.

The main reason why the prior art single cone bearing has failed tomaintain the attitude of rotating parts is that the bearing was providedonly with a load equal to the weight of its own, or even less than thatby using a magnetic bearing in order to avoid friction during theinitial and final periods of operation as disclosed in JapaneseLaid-open Utility Model Publication No. Hei. 06-004731. As has beenexplained above, a good balance is achieved between two forces of theaxial component of load capacity of the bearing versus the load.Therefore, a small load can only create a small load capacity, which isinsufficient to create forces for maintaining stable attitude ofrotating parts. In the air dynamic bearing of the present invention, alarge load is applied on the bearing by the magnetic attraction actedbetween the shaft and the sleeve. Therefore, the load capacity of thebearing, which counterbalances the load, can be set to a desired largevalue, whereby the stability of the attitude of rotating parts isimproved. The magnetic attraction may be varied case by case dependingon permissible level of NRRO, the size of the motor, and various otherconditions.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and advantages of the present invention willbecome clear from the following description with reference to theaccompanying drawings, wherein:

FIG. 1 is a cross sectional view showing an air dynamic bearing motoraccording to a embodiment of the present invention;

FIG. 2 illustrates the means for generating magnetic attraction and thelines of magnetic flux in the embodiment shown in FIG. 1;

FIG. 3(a) and FIG. 3(b) illustrate the bearing section in detail, FIG.3(a) being a plan view of a sleeve, and FIG. 3(b) being a crosssectional view of a shaft and the sleeve;

FIG. 4(a) illustrates a cross-section of the shaft and the sleeve, andcomponent forces of load capacity, and FIG. 4(b) illustrates thedistribution of pressure developed during the rotation;

FIG. 5 is a detailed cross sectional view of the bearing section havinga permanent magnet at one end of the shaft for limiting contact betweenthe shaft and the sleeve when they are stationary;

FIG. 6 is an explanatory view illustrating how the permanent magnet ofFIG. 5 is fitted in a predetermined position;

FIG. 7 is a cross sectional view of a modified construction of theembodiment in which a channel is formed through the shaft;

FIG. 8(a) illustrates a cross-section of the bearing section having acrown, with a graph showing the pressure distribution, and

FIG. 8(b) illustrates how the load capacity acts on the rotary sectionwhen it is offset from the center;

FIG. 9(a) and FIG. 9(b) are detailed views of the bearing section havinga modified construction wherein grooves are formed on both oppositesurfaces of the shaft and the sleeve, FIG. 9(a) being a plan view of thesleeve, and FIG. 9(b) being a cross sectional view of the shaft and thesleeve.

FIG. 10(a) and FIG. 10(b) are explanatory views illustrating how aring-shaped member can be axially adjusted, FIG. 10(a) being a crosssectional view of the bearing section, and FIG. 10(b) being an enlargedcross sectional view of the vicinity of the ring-shaped member;

FIG. 11 is a cross sectional view of a modified construction of theembodiment in which both ends of the shaft are fixed;

DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of an air dynamic bearing motor according to thepresent invention will be hereinafter described with reference to theaccompanying drawings.

FIG. 1 is a cross sectional view of a fluid dynamic bearing motoraccording to a embodiment of the present invention. A shaft 11 has adiminishing conical taper, and a sleeve 12 arranged opposite the shaft11 has a conical concavity. A permanent magnet 35 is provided within theshaft 11, so as to generate magnetic attraction between itself and thetop of the sleeve 12 which is made of a magnetic material.

Reference numeral 39 illustrates breathing holes which leak compressedair from the top 14 of the cone. The size of channel 39 are enough smallto adjust the flow resistance such that pressure remains at the top 14of the cone, whereby the sleeve 12 can fly up swiftly at the time ofstart-up. Reference numeral 90 represents magneteic material which actas magnetic shield. Rotary section is composed of the sleeve 12, a rotormagnet 46, and a magnet disk 85, and fixed section is composed of theshaft 11, a base 43, a stator core 49, coils 52, and others.

The bearing section is constituted by the shaft 11, the sleeve 12, and aseries of herringbone grooves, to be described later, provided in one ofthe conical taper surfaces 13 of the shaft 11 and the sleeve 12. Thegrooves serve to pump the air toward their center to increase thepressure of the air. The load capacity thereby created is in inverseproportion to the size of the clearance between the shaft 11 and sleeve12. Therefore, the clearance size is determined such that the axialcomponents of the load capacity and the above-mentioned magneticattraction are in equilibrium, while radial components of the loadcapacity are used for the centering of the sleeve 12. Accordingly, themagnetic attraction, which determines the load capacity, is set so thatthe load capacity is large enough to support the rotary section duringrotation. The clearance, accordingly, is approximately severalmicrometers wide. When the apical conical angle of the bearing sectionis large, the axial components of the load capacity may be given moreconsideration, while the radial components play a more important rolewhen the apical conical angle is small. In this embodiment, the angle ofthe cone apex is slightly smaller than 60° so as to give more weight tothe radial components to ensure precise centering of the sleeve.

The stator core 49 and the coils 52 cooperate with the rotor magnet 46to drive the rotary section. The rotary section further includes amagnetic or optical disk or the like carried thereon as a load. Theforce applied to the interface between the shaft 11 and the sleeve 12varies depending on the manner in which the memory device is installedin a normal state or inverted state. That is, if the device is set in anormal state, the bearing receives the weight of the movable parts inaddition to the magnetic attraction. If the device is set in an invertedstate, the bearing receives a load less than the magnetic attractionbecause the weight of the movable parts is subtracted therefrom. Inlight of this, the magnetic attraction should be approximately threetimes larger than the weight of the movable parts, which has empiricallybeen confirmed to ensure stable rotating attitude of the rotary section.If the magnetic attraction is increased so as to create accordinglylarger load capacity, precession of the shaft can further be restrictedand its attitude can be made more stable. On the other hand, it has beenascertained that such increase in the magnetic attraction causes thesliding friction to become larger at the time of starting up or stoppingthe motor, resulting in shorter operable life of the bearing. Therefore,in the case of the air dynamic bearing motor for a small magnetic diskdevice, magnetic attraction should be approximately five times largerthan the weight of the movable parts, which is the sum of the weight ofthe rotary section and the load weight. Such settings may be determinedcase by case depending on the required precision for the rotatingattitude of rotary section.

FIG. 2 illustrates a cross-section of the shaft 11, permanent magnet 35,sleeve 12 and others, together with lines of magnetic flux. The shaft 11is made of a non-magnetic material while having the permanent magnet 35inside, which is made of highly magnetic rare earth. Provided that thebearing section has about 20 mm diameter, the design value allotted tothe diameter of the permanent magnet 35 is up to 15 mm. Since the top ofthe shaft 11 and the sleeve 12 are arranged with a very small clearanceof about 20 micrometers therebetween, magnetic attraction remainsconstant irrespective of the variations in this clearance. Thus thetolerance in machining and assembling can be set larger. Referencenumeral 55 indicates the direction of magnetization of the permanentmagnet 35. The distal end of the permanent magnet 35 is formed sphericalso as to concentrate the magnetic flux. The magnetic flux 56 thusintensified enters the truncated cone top end of the sleeve 12, itpasses through the sleeve 12 and returns to the other end of thepermanent magnet 35. The magnetic flux returning to the end of thepermanent magnet 35 flies a long distance and over a large area and thusis distributed and low in intensity.

Apart from the structure shown in FIG. 2, magnetic attraction could bedeveloped using the rotor magnet 46 and the stator 49 axially offsetfrom each other, or the rotor magnet 46 and the magnetic piece arrangedbelow the rotor magnet. However, the former has a disadvantage that itproduces vibration, and the latter causes an increase in consumedcurrent because of the Foucault current developed in the magnetic piece.The magnetic attraction generating means in this embodiment can resolveall these problems encountered by the above-mentioned other mechanisms.

FIG. 3(a) and FIG. 3(b) illustrate the structure of the bearing sectionof the embodiment shown in FIG. 1 in more detail. FIG. 3(a) is a planview of the sleeve 12, and FIG. 3(b) is a cross sectional view of theshaft 11 and the sleeve 12. As shown in FIG. 3(a), a series ofherringbone grooves 18 is provided on the taper surface 13 of the sleeve12. The grooves 18 are V-shaped shallow recesses of about severalmicrometers depth. When the motor rotates, the grooves 18 pump the airfrom the outer and inner peripheral sides toward their central pointedends to increase the pressure of the air, so as to lift the sleeve 12from the shaft 11 and support it in a flying state. In this embodiment,the grooves are formed so that the pumping capacity from the outerperipheral side toward the inner peripheral side is larger than thatfrom the inner peripheral side toward the outer peripheral side, wherebythe pumping capacity towards the inner peripheral side remains and thepressure of the air on the inner peripheral side can be increasedswiftly when starting up the motor, so as to decrease the slidingfriction between the shaft 11 and the sleeve 12. The grooves 18illustrated in FIG. 3(a) have larger groove length on the innerperipheral side, but this does not contradict the description in theforegoing, since the pumping capacity is determined by the diminishingdegree of the circumferential length of the grooves and the radiallength of the grooves.

FIG. 4(a) and FIG. 4(b) illustrate the distribution of pressuredeveloped in the air when the motor rotates and the component forces ofthe load capacity applied to the interface between the shaft 11 and thesleeve 12 in accordance with the pressure distribution. These drawingsare given in explanation of how the rotating attitude of the sleeve isself-adjusted.

FIG. 4(b) shows various features 62, 63, 64, and 65 of the pressuredistribution of the air caused by the grooves 18 in operation. They-axis 60 represents pressure, while the x-axis 61 indicates radialcoordinates corresponding to FIG. 4(a). The pressure reaches a highestpoint 63, 65 at positions substantially corresponding to the pointedends of the V-shaped grooves 18. The drawing shows the pressuredistribution without the influence of the atmospheric pressure, andtherefore the pressure 62 at an outer peripheral point is almost zero.On the other hand, the pressure 64 at an inner peripheral point ishigher than the atmospheric pressure, because the grooves 18 are formedto have larger pumping capacity towards the inner peripheral side.

FIG. 4(a) shows a cross-section of the shaft 11 and the sleeve 12.Reference numerals 67, 68 represent the load capacity created as thepressure in the air increases. It should be noted that such a loadcapacity is created at each one of the several circumferentially locatedpoints, but only two of these are shown in a cross-section for the easeof explanation.

Reference numerals 69, 71 represent the axial components of the loadcapacity 67, 68, respectively. Reference numerals 70, 72 representrespective radial components thereof. Since the load capacity 67, 68 issubstantially in inverse proportion to the size of the clearance betweenthe shaft 11 and the sleeve 12, the clearance is determined such thatthe axial components 69, 71 and the magnetic attraction between therotary section and the fixed section are in equilibrium. The radialcomponents 70, 72 act in opposite directions so that they counterbalanceeach other, whereby the sleeve 12 is centered.

The load capacity 67, 68 acts vertically to the conical surfaces. Thus,it acts on the sleeve 12 as moment, i.e., the distance L multiplied bythe load capacity 67, 68, where L is the distance from an imaginaryfulcrum 66 corresponding to the cone apex and the point from which theload capacity 67, 68 acts. The moment resulting from the load capacity67, 68 acts in reverse directions, and because the load capacity 67, 68is substantially in inverse proportion to the nearby clearance betweenthe shaft 11 and the sleeve 12, the moment caused by the load capacity67, 68 acts around the fulcrum 66 as a position adjusting force for thesleeve 12, counterbalancing each other to equalize the clearance betweenthe shaft 11 and the sleeve 12. Thereby, the attitude of the sleeve 12is maintained upright, and its precession is restricted.

It will be understood from FIG. 4(a) that the magnetic attraction 83acting from the top end of the shaft 11 serves as the moment to adjustthe rotating attitude jointly with the load capacity 67, 68. Thus thestructure in which one end of the shaft has magnetic attraction can moreadvantageously help maintain the stable attitude of rotary section.

Furthermore, the motor according to the invention is low in respect ofbearing loss. Bearing loss of the air dynamic bearing is mainly causedby friction between the surfaces of the shaft 11 and sleeve 12 and theair in small clearances where the grooves exist. The bearing accordingto the invention has only a series of grooves, which is a practicalminimum, and thereby can achieve a reduction in required current.

The moment which acts on the sleeve 12 to maintain its attitude isdefined by the product which is obtained by multiplying the distance Lby the load capacity 67, 68 as noted above. Therefore, there is no needto provide two series of grooves with a large span therebetween in anaxial direction as in the prior art. The motor according to theinvention needs only one series of grooves 18, therefore the structureis more simple and thinner than the prior art.

For the material of the bearing components such as shaft and sleeve, anyof the materials such as ceramics stainless steel or copper alloy whichhave commonly been used for the fluid dynamic bearing can be used.Preferably, a thin film of nickel, titanium, diamond-like-carbon, ormolybdenum disulfide should be formed on one of the conical tapersurfaces, so as to decrease the friction at the time of starting up andstopping the motor.

Regarding the manufacturing method of the bearing components, not tomention the shaft having a convex shape, the sleeve, although having aconcave shape, it can be easily released, because its tapered top isopened. Therefore they both can be formed at one time including thegrooves, by any known techniques such as press molding or injectionmolding. Accordingly, the bearing components can also be made of aceramics or sintered alloy by molding, or of a resin material havingsuperior antifriction properties such as polyphenyl sulfide resin (PPS)containing carbon fiber by molding, whereby a reduction in productioncost is achieved.

FIG. 5 and FIG. 6 are detailed views of the bearing section illustratinghow the permanent magnet prevents the shaft and the sleeve from makingsurface contact with each other when they are stationary. As shown inFIG. 5, the permanent magnet 35 is provided at the top end of the shaft11, such as to contact the inside top limit of the conical sleeve 12when stationary. The dotted lines 12 a illustrate the position of thesleeve when stationary, while the solid lines indicate the position ofthe sleeve 12 when rotating. The permanent magnet 35 protrudes by apredetermined amount such that f≧d, where d is the distance between thetop of the permanent magnet 35 and the inside top limit of the sleeve12, and f is the axial flying height of the sleeve 12 from the shaft 11measured at conical surfaces. To be specific, the permanent magnet 35 isprotruded so that f−d is about 5 micrometers if the flying height isaround 10 micrometers. Thus the top of the sleeve 12 flies up from thepermanent magnet 35 at least about 5 micrometers during rotation, whileits conical surface flies up to an axial height of about 10 micrometers,maintaining a stable rotating attitude.

Conical bearings have a potential risk that the shaft fits into thesleeve, increasing the friction therebetween, resulting in start-upfailure. This is caused by various factors such as the intensity ofmagnetic attraction, the apical conical angles, and the hardness of thematerial making up the shaft and sleeve, correlating with each other.Small motors to which the present invention is applied are relativelyfree of such troubles, but the structure shown in FIG. 5 further ensuresthat no such troubles will occur.

FIG. 6 is given in explanation of how the permanent magnet shown in FIG.5 is adjusted in position. The permanent magnet 35 is initially fittedin the cylinder 32 inside the shaft 11 with clearance so as to bemovable, but firmly enough to overcome the magnetic attraction. Forassembling the permanent magnet 35, it is placed upon the shaft 11 asbeing protruded substantially therefrom, and the sleeve 12 is coupledthereon. Pressure that is larger than the magnetic attraction is thenapplied to the sleeve 12 and the shaft 11 so that the permanent magnet35 contacts the inside top limit of the sleeve 12, until the shaft 11and the sleeve 12 make surface contact with each other on their conicalsurfaces and the truncated cone apex of the sleeve 12 is resilientlydeformed. The dotted lines 12 b indicate the deformed sleeve underpressure, while the solid lines indicate the sleeve 12 having restoredto have its initial truncated conical apex, after the pressure has beenremoved. As the sleeve 12 resiliently returns into its initial shape, aclearance is created between the conical surfaces of the shaft 11 andthe sleeve 12. A plate spring may be arranged on the inside top limit ofthe sleeve 12 instead of utilizing the resilient deformation of the topof the sleeve.

After the position alignment, the permanent magnet 35 should preferablybe fixed in position by bonding or welding, so as to withstand largeshocks. Further, it is preferable to provide antifriction measures onthe top of the permanent magnet 35 and the opposite inside top limit ofthe sleeve 12 such as application of a ceramic material or platingtreatment, so as to ensure stable performance over a long time.

Single cone bearings have the characteristics that even when the shaftand the sleeve have slightly different diameters, they still can faceeach other at given axial positions, whereby the tolerance of theirdimensions can be made large, offering the advantage of lower cost. Thepermanent magnet 35 shown in FIG. 5 could initially be fixed to theshaft 11, but in that case the diameters of the shaft 11 and the sleeve12 and the protruding amount of the permanent magnet 35 must preciselybe controlled. If the demands for the performance of the fluid dynamicbearing motor in regard to inhibition of NRRO are relatively low, thensuch control of dimensions could easily be achieved, while it is not ifthe demands are high. Thus the total cost would be lower with thestructure wherein the permanent magnet allows itself to be positionallyadjusted as in this embodiment.

FIG. 7 shows another modified construction of the embodiment having achannel 34 that runs through the shaft 11 from its truncated cone top 14to the outer periphery thereof. The channel 34 is provided forcirculating the air compressed towards the top 14 of the shaft 11 to theoutside of the cone. The channel 34 is filled with fibrous or porousmaterial to adjust the flow resistance such that pressure remains at thetop 14 of the cone, whereby the sleeve 12 can fly up swiftly at the timeof start-up, and whereby shock-absorbing effects are achieved because ofthe compressed air that escapes and adjusts the damping level. Moreover,galls produced on the sliding parts can be removed with the structure ofthis example.

Also the channel 34 can run through to the outside of the motor viafiltration material such as fibrous or porous material. The referencenumeral 38 shows the permanent magnet with a small groove at the side.

FIG. 8 shows a modified embodiment in which has herringbone grooves andcrown in the conical surface. The herringbone grooves in the conicalsurface are formed to have flat region in central parts as shown in FIG.8(a). While the grooves 20, 21 on both sides of the crown are shown inthe cross sectional view so that their positions are more clearlyunderstood, they are actually formed on the surface of the conical shaft11, having a several micrometers depth. The shaft 11 has a slightlybulging crown 19 on its conical surface so as to have a flat band regionwhere the bearing clearance is minimum. Correspondingly, acircumferential groove 40 of about 10 micrometers depth is provided inthe sleeve 12 opposite the flat band region formed by the crown 19.Specific dimensions of the crown 19 may differ case by case depending onvarious conditions, but basically they are set such that the bearingclearance at the outermost periphery of the conical shaft 11 and thesleeve 12 is several micrometers larger than that in the flat bandregion. With this construction, even if the apical conical angles of theshaft 11 and the sleeve 12 are not precisely in conformity with eachother, edge contacts at the inner and outer peripheries can beprevented. Therefore, the machining tolerance of the components can bemade larger.

The herringbone grooves are made up of two types of spiral grooves forpumping in and pumping out purposes. In other words, pumping-out spiralgrooves 20 are positioned on the inner peripheral side, while pumping-inspiral grooves 21 are arranged on the outer peripheral side, with thecrown 19 for making the bearing clearance minimum positionedtherebetween. The number of grooves per one round, the inclination angleof the grooves, and other features of the grooves can suitably be setaccording to their purposes.

FIG. 8(a) shows the pressure distribution observed during the operationof the bearing having the above-described grooves. The y-axis 73indicates axial coordinates, while the x-axis 74 represents pressure.Reference numerals 75, 76, 77, 78, and 79 represent mean pressure valuesin a circumferential direction at respective axial positions. Thedrawing shows the pressure distribution without the influence of theatmospheric pressure, and therefore the pressure 75 at an outerperipheral point is zero. The pressure increases as denoted by thereference numeral 76 because of the grooves 21, and becomes constant inthe central region as indicated by the reference numeral 77. Thepressure decreases at a position where the grooves 20 are formed asindicated by the reference numeral 78. At the top 14 of the cone, thepressure is slightly higher than the atmospheric pressure as indicatedby the reference numeral 79.

The attitude of the rotary section is basically maintained by the highpressure 77 in the central region. A more specific account of theposition adjusting mechanism will be given below with reference to FIG.8(b). The pressure values 75, 76, 77, 78, and 79 in the pressuredistribution of FIG. 8(a) are mean values in circumferential directionsand they may locally vary if the sleeve 12 comes off-center or tiltswith respect to the shaft 11. FIG. 8(b) illustrates a state wherein thesleeve 12 is rotating as being inclined leftward at the upper partthereof and rightward at the lower part thereof with respect to theshaft 11. The load capacity, created by the grooves 20 in the centralregion where the clearance is made small by the crown 19, becomes unevenin the circumferential direction, i.e., the load capacity F11 on theright side becomes larger than the load capacity F12 on the left sidebecause the bearing clearance is smaller on the right side. Similarly,the pressure developed by the grooves 21 becomes uneven, the loadcapacity F21 on the right side being smaller than the load capacity F22on the left side. Here, the load capacity acts on the upper part of thesleeve 12 as moment of L1*(F11-F12), while it acts on the lower part ofthe sleeve 12 as moment of L2*(F21-F22), where L1, L2 are the distancesfrom an imaginary fulcrum 66 corresponding to the cone apex and therespective points from which the load capacity F11, F12, F21, F22 acts.The moment acts around the fulcrum 66 as a force to make the bearingclearance at respective points equal. It should be noted that thedescription given above is simplified and the moment actuallycounterbalances each other at all circumferential and axial points, notonly on the left and right sides.

In this way, by arranging a series of herringbone grooves on the conicalsurface with a small clearance region therebetween, a moment force isgenerated that acts on the rotary section to equalize the upper andlower clearances between the shaft 11 and the sleeve 12, therebyadjusting the rotating attitude of the rotary section. Thus theprecession is further restricted in the air dynamic bearing motor ofthis embodiment. When the sleeve 12 comes off center with respect to theshaft 11, the pressure in the lubricant locally increase because of thewedge effect in the intermediate small-clearance band region formed bythe crown 19. A delay from the time when the bearing clearance isreduced until the time when a large pressure is developed may inducehalf whirls or other unstable movements of the rotary section. This iswhy the circumferential groove 40 is provided, as it distributes thelocally collected lubricant in circumferential directions, therebyenhancing the position adjusting effect by the grooves and preventinghalf whirls.

FIG. 9(a) and FIG. 9(b) show the vicinity of the bearing sectionaccording to a further modified construction of the embodiment in whichgrooves are formed on both opposite surfaces of the bearing section.FIG. 9(a) shows a bearing surface of the sleeve 12 in a plan view. Asshown, the sleeve 12 has on its bearing surface a plurality ofherringbone grooves 18. FIG. 9(b) is a cross-section of the shaft andthe sleeve. The shaft 11 has a permanent magnet 35 inside for generatingmagnetic attraction. On its outer surface, a series of herringbonegrooves 22 is formed. The grooves 18 and 22 have a depth of aboutseveral micrometers, and grooves 22 on the surface of the shaft 11 andthose 18 on the sleeve 12 have different angular lengths in thecircumferential direction. In the specific example given in thesedrawings, the grooves 18 on the surface of the sleeve 12 have angularlengths of more than twice as large as that of the grooves 22 on theshaft 11 in the circumferential direction. The arrows 29, 30 indicatethe direction in which the sleeve 12 rotates.

Grooves pump the air when the bearing rotates to increase the pressurein the air. The increased pressure, which is substantially in inverseproportion to the bearing clearance, causes a force to act on the rotarysection to adjust its rotating attitude. Since the grooves are arrangedat circumferentially spaced positions, even if the sleeve comesoff-center with respect to the shaft and the bearing clearance becomeslocally small, there is a delay until the balance in the circumferentialpressure distribution is disturbed. This delay or time lag is inproportion to the angular length of the grooves in the circumferentialdirection. It is known that control systems with the time lag betweenthe change in the controlled variable and the control over the changeare susceptible to a resonant phenomenon, which, in the case of the airdynamic bearing, takes the form of precession, half whirl or otherunstable movements.

In order to avoid such unstable movements, for example, thecircumferential length of the grooves 18 may be varied so that the timelag is varied. However, if the angular lengths of only several groovesin one round are changed, the possibility of the position adjustingforce not acting evenly increases, or other problem may arise.Therefore, in this embodiment, the grooves on the shaft 11 and those onthe sleeve 12 are varied in their angular lengths in the circumferentialdirection so as to both achieve the circumferential evenness in theposition adjusting force which is created by the increased pressure inair, and the variety in the angular length of the grooves in thecircumferential direction. Machining of the grooves is generally noteasy and forming them on both bearing surfaces may lead to an increasein cost. However, the conical shaft 11 and the sleeve 12 in thisembodiment can both be produced by molding, and therefore such groovescan be provided without increasing cost. Thus an air dynamic bearingmotor with limited precession is realized.

FIG. 10(a) and FIG. 10(b) illustrate a modified construction of theembodiment wherein the ring-shaped member can be adjusted in axialdirections. FIG. 10(a) is a cross sectional view of the bearing section,and FIG. 10(b) is an enlarged cross sectional view of part 89 of thering-shaped member and other components. In this example, the sleeve 12has a protrusion 86 on its end, while the ring-shaped member 24 has acorresponding through hole to match this protrusion. The ring-shapedmember 24 is preliminarily coupled into the annular recess 26 around theshaft 12 and assembled to the sleeve 12. Access holes 25 are provided,through which the protrusion 86 and the through hole of the ring-shapedmember 24 are engaged with each other. Then, using a jig 88, the innerperiphery of the ring-shaped member 24 is abutted onto the end face 87of the annular recess 26. The ring-shaped member is thus coupled to theprotrusion 86 as being resiliently deformed.

In this assembling process, the ring-shaped member 24 is resilientlydeformed in an axial direction by about 20 micrometers, while beingcoupled to the protrusion 86 firmly. Thereby, axial displacement of therotary section is restricted to be about 20 micrometers even if it issubjected to large shocks. In the case of hard disk drives, there is astrong demand for restricting axial displacement of the magnetic disk toa minimum. By utilizing resilient deformation of the ring-shaped member24 as in this embodiment, such requirements can be met without higherdemands for the tolerance of various components. Alternatively, thering-shaped member 24 and the protrusion 86 may be joined after theassembly by bonding or welding to have a higher strength to withstandlarge impacts.

FIG. 11 illustrate another modified construction of the embodimentwherein both ends of shaft are fixed. In this embodiment, a smalldiameter shaft 36 is fixed at the center of the permanent magnet 35 andis protuded through the center hole 84 of the sleeve 12. Other featuresare the same with the embodiment shown in FIG. 1, so the explanationabout them is not repeated. In case of a thinner device, such smalldiameter shaft 36 can support the upper wall of the case.

Although the embodiments shown in above have been described as havingthe sleeve 12 and the hub formed in one piece, they may be separatecomponents and assembled together. Whether they should be produced inone piece or separately may be determined case by case so that the costis lower, taking into consideration the characteristics andspecifications required for each component. In the application of theinvention to a hard disk drive as has been shown in these embodiments,however, there are stringent specifications with regard to the heightand tilt of the install surface of the magnetic disk. Since these arestrongly affected by their positional relationship with the bearingsurface, it is more preferable to form the sleeve 12 and the hub in onepiece to achieve higher precision. The air dynamic bearing motoraccording to the present invention enables the integral structure of thesleeve and the hub and realizes a high-precision, low-cost motor.

According to an air dynamic bearing motor of the present invention, thebearing section has a simple structure wherein grooves are formed on aconical taper surface for increasing the air pressure and creating aload capacity, which is balanced with magnetic attraction. With thisstructure, the attitude of the rotary section in the bearing is madestable. The bearings can be mass-produced at low cost by molding, andthe total thickness of the motor can be reduced. The current requiredfor operating the motor is reduced. Therefore, the motor according tothe invention is particularly suitable for small, rotary memory devicessuch as magnetic or optical disk devices, or cooling fans for CPUs.

While there has been described what are at present considered to bepreferred embodiments of the present invention, it will be understoodthat various modifications may be made thereto, and it is intended thatthe appended claims cover all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:
 1. An air dynamic bearing motor, comprising: a shafthaving a diminishing conical taper surface; a sleeve having a conicalconcavity opposite the shaft; and means for generating magneticattraction between one end of the shaft and a cone apex of the sleeve,including a permanent magnet and a magnetic material, the magnet beingprovided in one of the shaft and the sleeve and the magnetic beingprovided in the other one thereof; wherein a plurality of grooves areformed on at least a conical taper surface of one of the shaft and thesleeve, the grooves being provided for creating load capacity when themotor rotates, whereby rotating parts of the motor are supported byaxial components of the load capacity balanced with said magneticattraction.
 2. The air dynamic bearing motor according to claim 1,wherein the magnetic attraction generating means comprises a permanentmagnet held within the shaft and a magnetic material provided in thesleeve, and wherein said permanent magnet is initially held movably witha stabilizing tolerance to overcome said magnetic attraction, and isadjustably fixed in position at the time of assembling such that thepermanent magnet makes contact with the sleeve when the motor isstationary, and wherein said permanent magnet and said sleeve arebrought out of contact when the motor is rotating, by a distance equalto or shorter than an axial flying height determined on conical surfacesof the shaft and sleeve.
 3. The air dynamic bearing motor according toclaim 1, wherein the magnetic attraction generating means comprises apermanent magnet held within the shaft and a magnetic material providedin the sleeve, the permanent magnet being assembled with the shaft suchthat said permanent magnet is initially held movably with a stabilizingtolerance to overcome said magnetic attraction as being substantiallyprotruded from one end of the shaft, and is pressed into the shaft by apressure larger than the magnetic attraction applied from both ends ofthe shaft and the sleeve to a predetermined position, where the coneapex of the sleeve or a plate spring interposed between the apex of thesleeve and the permanent magnet is resiliently deformed, wherein, whenthe motor is stationary the permanent magnet and the apex of the sleeveor the plate spring make contact with each other, and wherein saidpermanent magnet and the apex of the sleeve or the plate spring arebrought out of contact when the motor is rotating, by a distance equalto or shorter than an axial flying height determined on conical surfacesof the shaft and sleeve.
 4. The air dynamic bearing motor according toclaim 1, further including a crown of a predetermined number ofmicrometers provided on the conical taper surface of one of the shaftand the sleeve so as to make the clearance between the opposite tapersurfaces of the shaft and the sleeve be minimum at an axiallyintermediate region, wherein the grooves are spiral grooves and providedon one or both sides of said axially intermediate region where theclearance between the shaft and the sleeve is minimum, for pumping theair towards said intermediate region.
 5. The air dynamic bearing motoraccording to claim 4, further including a circumferential grooveprovided on the conical taper surface of at least one of the shaft andthe sleeve where the clearance therebetween is minimum because of thecrown.
 6. The air dynamic bearing motor according to claim 1, whereinthe grooves are formed on both opposite conical taper surfaces of theshaft and the sleeve at radially opposite positions, the grooves havingdifferent angular lengths from each other in a circumferentialdirection.
 7. The air dynamic bearing motor according to claim 1,further including a ring-shaped member fixed to an end of said sleeve,and an annular recess provided in the fixed part opposite to the sleeveand making engagement with the ring-shaped member so as to restrictaxial movable distance of the rotating parts.
 8. The air dynamic bearingmotor according to claim 7, wherein the ring-shaped member is fixed tothe end of the sleeve by any one of interfitting, bonding, and welding,access holes being provided in one of a fixed member opposite said theend of the sleeve for enabling the fixing of the ring-shaped member tobe performed.
 9. The air dynamic bearing motor according to claim 8,further including a means for establishing coupling engagement betweenthe ring-shaped member and the sleeve end, wherein the ring-shapedmember is coupled to one end of the sleeve with a free peripheralportion thereof being pressed, through the access hole, to beresiliently deformed and abutted onto an end face of said annularrecess, and wherein said resilient deformation of the ring-shaped memberdetermines a permissible range of the axial displacement of the rotatingparts.